Heat transfer labels are commonly used in the decorating and/or labeling of commercial articles, such as, and without limitation, containers for beverages, essential oils, detergents, adverse chemicals, and health and beauty aids. Heat transfer labels are desirably resistant to abrasion and chemical effects in order to avoid a loss of label information and desirably possess good characteristics of adhesion to the articles to which they are affixed.
Heat transfer labels are multilayered laminates, with each layer having its own function. For example, heat transfer labels generally include an adhesive layer, an ink design layer, and a release layer. The release layer may be a wax release layer, and is often directly adjacent a carrier sheet, such as on a roll or web of labels. Thus, in such an example, the label may be thought to include a “support portion” (e.g., carrier sheet and release layer and a “transfer portion” (i.e., ink design layer and adhesive layer). When subjected to heat, the wax release layer melts, thereby allowing the transfer portion to be separated from the carrier sheet, and the adhesive layer adheres the ink design layer to an article being labeled. Alternatively, all or part of the wax release layer may transfer as well, to provide protection to the ink design layer. Additionally or alternatively, the labels may include a separate protective layer overlying the ink design layer to protect the ink design layer from abrasion.
More specifically, in the heat transfer labeling process, the label-carrying sheet is subjected to heat, and the label is pressed onto an article with the ink design layer making direct contact with the article. As the paper sheet is subjected to heat, the wax layer begins to melt so that the paper sheet can be released from the ink design layer. (And, as described above, a portion of the wax layer may be transferred with the ink design layer and a portion of the wax layer may remain with the paper sheet.) After transfer of the ink design layer to the article, the paper sheet is removed, leaving the ink design layer firmly affixed to the article. In an alternate embodiment, where the wax layer also transfers, the wax layer thus may serve two purposes: (1) to provide release of the ink design layer from the sheet upon application of heat to the sheet, and (2) to form a protective layer over the transferred ink design layer. After transfer of the label to the article, the transferred wax release layer may be subjected to a postflaming technique which enhances the optical clarity of the layer (thereby enabling the ink design layer therebeneath to be better observed) and which enhances the protective properties of the transferred wax layer.
Such heat transfer labels have been used to decorate a variety of articles, such as polyethylene, polypropylene, PET, and acrylonitrile articles. For example, such articles may include high-density polyethylene (HDPE) containers, low-density polyethylene (LDPE) containers, and polypropylene containers. One example of a heat transfer label that has been used to decorate polyethylene (PE) containers includes a paper carrier sheet overcoated with a wax release layer (approximately 6-8 lbs. wax/3,000 square feet of paper carrier web). A protective lacquer layer including a polyester resin is printed on the wax release layer. An ink design layer including a polyamide resin is printed on the protective lacquer layer. A heat-activatable adhesive layer including a polyamide resin is printed on the ink design layer.
One disadvantage associated with the use of the aforementioned label, and similar heat transfer labels, on polyethylene, polypropylene, PET, and/or acrylonitrile, is that the label will not adhere to a polyethylene, polypropylene, PET, or acrylonitrile surface unless the surface has previously been treated by some oxidizing technique. It is known to those skilled in the art that to effect a bond between article surface and an adhesive including a polyamide, the article surface needs to be oxidized first, as described above. This also is the case for adhesives including chlorinated polyolefins. Such adhesives also need the article surface to be oxidized in order to effectively bond to the article, as described above. Typical oxidizing techniques include flaming the polyethylene, polypropylene, PET, or acrylonitrile surface with an oxidizing flame. Without wishing to be limited to any particular theory as to why pretreatment of the polyethylene, polypropylene, PET, or acrylonitrile surface is necessary for the aforementioned label to adhere thereto, it is believed that untreated polyethylene, untreated polypropylene, untreated PET, or untreated acrylonitrile is a low energy surface made up primarily of hydrocarbons, whereas oxidized or treated polyethylene, polypropylene, PET, or acrylonitrile is a relatively higher energy surface which additionally includes ketones, carboxylic acid groups, etc. Accordingly, because the pretreated polyethylene, polypropylene, PET, or acrylonitrile surface is a higher energy surface than the untreated polyethylene, polypropylene, PET, or acrylonitrile surface, it is more receptive to binding to the adhesive layer of the label. However, pretreatment of the article results in increased time, equipment, and cost in labeling the article.
Thus, it would be desirable to provide a heat transfer label that is particularly well suited for use on untreated polyethylene, polypropylene, PET, or acrylonitrile surfaces, such as untreated high, medium, or low density polyethylene surfaces and/or untreated high, medium, or low density polypropylene surfaces.